1. Field of the Invention
This invention relates to current-to-position transducers used in industrial process control instrumentation. More particularly, this invention relates to such transducers which are especially adapted to produce a corresponding pneumatic pressure output signal or proportionally control by pneumatic means the position of a driven element, for example process control valve stem.
2. Description of the Prior Art
The conversion of electric input signals, which represent various control variables, into pneumatic outputs that may be used as set-point inputs for pneumatically operated instruments or for directly positioning the stem of a valve is well known in the process control instrumentation art. Throughout the years, numerous instruments have been developed for these purposes.
In certain of the prior art instruments, the input electrical current is first converted into an equivalent force either by means of a voice coil or a flat armature torque motor. The resultant force is then balanced across a pivoted element by a bellows. A pneumatic flapper-nozzle system maintains the pivoted element at null by applying the required offsetting pressure to the bellows. Hence an equivalence is established between the input current and the output pressure.
This method of pneumatically generating an output signal by opposing torques is commonly referred to as a force-balance system. Such a system is characterized in that little palpable motion occurs among the various operative components (i.e., nozzle-flapper, pivoted element, and bellows). Typically the range of movement of these components is in the order of 0.001 inch. Instruments of this type are commercially available from The Foxboro Company (Model 69TA) and Fisher Governor Company (Type 546).
Notwithstanding prior development efforts represented by the marketing of numerous prior instruments in this field, these instruments have not been wholly satisfactory and are more costly to build. In particular, prior art instruments of the force-balance type possess limited flexibility for use in other process control applications.